Researchers go 'outside the box' to delineate major ocean currents

For the first time, University of Rochester researchers and collaborators have quantified the energy of ocean currents larger than 1,000 kilometers. In the process, they discovered that the most energetic is the Antarctic Circumpolar Current, some 9,000 kilometers in diameter.

The U.S. National Science Foundation-supported team, led by Hussein Aluie, used the same coarse-graining technique developed by his lab to document energy transfer at the other end of the scale, during the "eddy killing" that occurs when wind interacts with temporary, circular currents less than 260 kilometers in size.

The results, reported in Nature Communications, show how the coarse-graining technique can provide a new window for understanding oceanic circulation in all its multiscale complexity, says lead author Benjamin Storer. That gives researchers an opportunity to better understand how ocean currents function as a key moderator of the Earth's climate system.

The team also includes researchers from the University of Rome Tor Vergata, the University of Liverpool and Princeton University.

Traditionally, scientists interested in climate and oceanography have picked boxes in the ocean 500 to 1,000 square kilometers in size. These box regions, which were assumed to represent the global ocean, were then analyzed using a technique called Fourier analysis, Aluie says.

"The problem is, when you pick a box, you are already limiting yourself to analyzing what's in that box," Aluie says. "You miss everything at a larger scale. What we are saying is, we don't need a box; we can think outside the box."

When the researchers use the coarse-graining technique to "blur" satellite images of global circulation patterns, for example, they find that they "gain more by settling for less," Aluie says. "It allows us to disentangle different-sized structures of ocean currents in a systematic way."

He draws an analogy to removing your eyeglasses, then looking at a very crisp, detailed image. It will appear to be blurred. But as you look through a succession of increasingly stronger eyeglasses, you will often be able to detect various patterns at each step that would otherwise be hidden in the details.

In essence, that is what coarse-graining allows the researchers to do: quantify various structures in ocean current and their energy "from the smallest, finest scales to the largest," Aluie says.

"This new approach lets us separate motions into different scales with fewer complications than previous ones," says Baris (Mete) Uz, a program director in NSF's Division of Ocean Sciences. "That may seem like a simple statistical advance, but such precise and practical definitions have a big impact on how we think of the ocean as a system."